Subject: Re: Multicast packet loss

From: Eric Dumazet <[email-masked]>
Date: Mon, 16 Mar 2009 23:22:01 +0100

David Miller a écrit :
> From: Eric Dumazet <[email-masked]>
> Date: Fri, 13 Mar 2009 23:30:31 +0100
> 
>> David Miller a écrit :
>>>> Also, when an event was queued for later invocation, I also needed to keep
>>>> a reference on "struct socket" to make sure it doesnt disappear before
>>>> the invocation. Not all sockets are RCU guarded (we added RCU only for 
>>>> some protocols (TCP, UDP ...). So I found keeping a read_lock
>>>> on callback was the easyest thing to do. I now realize we might
>>>> overflow preempt_count, so special care is needed.
>>> You're using this in UDP so... make the rule that you can't use
>>> this with a non-RCU-quiescent protocol.
>> UDP/TCP only ? I though many other protocols (not all using RCU) were
>> using sock_def_readable() too...
> 
> Maybe create a inet_def_readable() just for this purpose :-)

Here is the last incantation of the patch, that of course should be
split in two parts and better Changelog for further discussion on lkml.

We need to take a reference on sock when queued on a softirq delay
list. RCU wont help here because of SLAB_DESTROY_BY_RCU thing :
Another cpu could free/reuse the socket before we have a chance to
call softirq_delay_exec()

UDP & UDPLite use this delayed wakeup feature.

Thank you

[PATCH] softirq: Introduce mechanism to defer wakeups

Some network workloads need to call scheduler too many times. For example,
each received multicast frame can wakeup many threads. ksoftirqd is then
not able to drain NIC RX queues in time and we get frame losses and high
latencies.

This patch adds an infrastructure to delay work done in
sock_def_readable() at end of do_softirq(). This needs to
make available current->softirq_context even if !CONFIG_TRACE_IRQFLAGS

Signed-off-by: Eric Dumazet <[email-masked]>
---
 include/linux/interrupt.h |   18 +++++++++++++++
 include/linux/irqflags.h  |   11 ++++-----
 include/linux/sched.h     |    2 -
 include/net/sock.h        |    2 +
 include/net/udplite.h     |    1
 kernel/lockdep.c          |    2 -
 kernel/softirq.c          |   42 ++++++++++++++++++++++++++++++++++--
 lib/locking-selftest.c    |    4 +--
 net/core/sock.c           |   41 +++++++++++++++++++++++++++++++++++
 net/ipv4/udp.c            |    7 ++++++
 net/ipv6/udp.c            |    7 ++++++
 11 files changed, 125 insertions(+), 12 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 9127f6b..a773d0c 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -295,6 +295,24 @@ extern void send_remote_softirq(struct call_single_data *cp, int cpu, int softir
 extern void __send_remote_softirq(struct call_single_data *cp, int cpu,
 				  int this_cpu, int softirq);
 
+/*
+ * softirq delayed works : should be delayed at do_softirq() end
+ */
+struct softirq_delay {
+	struct softirq_delay	*next;
+	void 			(*func)(struct softirq_delay *);
+};
+
+int softirq_delay_queue(struct softirq_delay *sdel);
+
+static inline void softirq_delay_init(struct softirq_delay *sdel,
+				      void (*func)(struct softirq_delay *))
+{
+	sdel->next = NULL;
+	sdel->func = func;
+}
+
+
 /* Tasklets --- multithreaded analogue of BHs.
 
    Main feature differing them of generic softirqs: tasklet
diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 74bde13..30c1e01 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -13,19 +13,21 @@
 
 #include <linux/typecheck.h>
 
+#define softirq_enter()	do { current->softirq_context++; } while (0)
+#define softirq_exit()	do { current->softirq_context--; } while (0)
+#define softirq_context(p)	((p)->softirq_context)
+#define running_from_softirq()  (softirq_context(current) > 0)
+
 #ifdef CONFIG_TRACE_IRQFLAGS
   extern void trace_softirqs_on(unsigned long ip);
   extern void trace_softirqs_off(unsigned long ip);
   extern void trace_hardirqs_on(void);
   extern void trace_hardirqs_off(void);
 # define trace_hardirq_context(p)	((p)->hardirq_context)
-# define trace_softirq_context(p)	((p)->softirq_context)
 # define trace_hardirqs_enabled(p)	((p)->hardirqs_enabled)
 # define trace_softirqs_enabled(p)	((p)->softirqs_enabled)
 # define trace_hardirq_enter()	do { current->hardirq_context++; } while (0)
 # define trace_hardirq_exit()	do { current->hardirq_context--; } while (0)
-# define trace_softirq_enter()	do { current->softirq_context++; } while (0)
-# define trace_softirq_exit()	do { current->softirq_context--; } while (0)
 # define INIT_TRACE_IRQFLAGS	.softirqs_enabled = 1,
 #else
 # define trace_hardirqs_on()		do { } while (0)
@@ -33,13 +35,10 @@
 # define trace_softirqs_on(ip)		do { } while (0)
 # define trace_softirqs_off(ip)		do { } while (0)
 # define trace_hardirq_context(p)	0
-# define trace_softirq_context(p)	0
 # define trace_hardirqs_enabled(p)	0
 # define trace_softirqs_enabled(p)	0
 # define trace_hardirq_enter()		do { } while (0)
 # define trace_hardirq_exit()		do { } while (0)
-# define trace_softirq_enter()		do { } while (0)
-# define trace_softirq_exit()		do { } while (0)
 # define INIT_TRACE_IRQFLAGS
 #endif
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8c216e0..5dd8487 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1320,8 +1320,8 @@ struct task_struct {
 	unsigned long softirq_enable_ip;
 	unsigned int softirq_enable_event;
 	int hardirq_context;
-	int softirq_context;
 #endif
+	int softirq_context;
 #ifdef CONFIG_LOCKDEP
 # define MAX_LOCK_DEPTH 48UL
 	u64 curr_chain_key;
diff --git a/include/net/sock.h b/include/net/sock.h
index 4bb1ff9..0160a83 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -260,6 +260,7 @@ struct sock {
 	unsigned long	        sk_lingertime;
 	struct sk_buff_head	sk_error_queue;
 	struct proto		*sk_prot_creator;
+	struct softirq_delay	sk_delay;
 	rwlock_t		sk_callback_lock;
 	int			sk_err,
 				sk_err_soft;
@@ -960,6 +961,7 @@ extern void *sock_kmalloc(struct sock *sk, int size,
 			  gfp_t priority);
 extern void sock_kfree_s(struct sock *sk, void *mem, int size);
 extern void sk_send_sigurg(struct sock *sk);
+extern void inet_def_readable(struct sock *sk, int len);
 
 /*
  * Functions to fill in entries in struct proto_ops when a protocol
diff --git a/include/net/udplite.h b/include/net/udplite.h
index afdffe6..7ce0ee0 100644
--- a/include/net/udplite.h
+++ b/include/net/udplite.h
@@ -25,6 +25,7 @@ static __inline__ int udplite_getfrag(void *from, char *to, int  offset,
 /* Designate sk as UDP-Lite socket */
 static inline int udplite_sk_init(struct sock *sk)
 {
+	sk->sk_data_ready = inet_def_readable;
 	udp_sk(sk)->pcflag = UDPLITE_BIT;
 	return 0;
 }
diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 06b0c35..9873b40 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -1807,7 +1807,7 @@ print_usage_bug(struct task_struct *curr, struct held_lock *this,
 	printk("%s/%d [HC%u[%lu]:SC%u[%lu]:HE%u:SE%u] takes:\n",
 		curr->comm, task_pid_nr(curr),
 		trace_hardirq_context(curr), hardirq_count() >> HARDIRQ_SHIFT,
-		trace_softirq_context(curr), softirq_count() >> SOFTIRQ_SHIFT,
+		softirq_context(curr), softirq_count() >> SOFTIRQ_SHIFT,
 		trace_hardirqs_enabled(curr),
 		trace_softirqs_enabled(curr));
 	print_lock(this);
diff --git a/kernel/softirq.c b/kernel/softirq.c
index bdbe9de..91a1714 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -158,6 +158,42 @@ void local_bh_enable_ip(unsigned long ip)
 }
 EXPORT_SYMBOL(local_bh_enable_ip);
 
+
+#define SOFTIRQ_DELAY_END (struct softirq_delay *)1L
+static DEFINE_PER_CPU(struct softirq_delay *, softirq_delay_head) = {
+	SOFTIRQ_DELAY_END
+};
+
+/*
+ * Caller must disable preemption, and take care of appropriate
+ * locking and refcounting
+ */
+int softirq_delay_queue(struct softirq_delay *sdel)
+{
+	if (!sdel->next) {
+		sdel->next = __get_cpu_var(softirq_delay_head);
+		__get_cpu_var(softirq_delay_head) = sdel;
+		return 1;
+	}
+	return 0;
+}
+
+/*
+ * Because locking is provided by subsystem, please note
+ * that sdel->func(sdel) is responsible for setting sdel->next to NULL
+ */
+static void softirq_delay_exec(void)
+{
+	struct softirq_delay *sdel;
+
+	while ((sdel = __get_cpu_var(softirq_delay_head)) != SOFTIRQ_DELAY_END) {
+		__get_cpu_var(softirq_delay_head) = sdel->next;
+		sdel->func(sdel);	/*	sdel->next = NULL;*/
+		}
+}
+
+
+
 /*
  * We restart softirq processing MAX_SOFTIRQ_RESTART times,
  * and we fall back to softirqd after that.
@@ -180,7 +216,7 @@ asmlinkage void __do_softirq(void)
 	account_system_vtime(current);
 
 	__local_bh_disable((unsigned long)__builtin_return_address(0));
-	trace_softirq_enter();
+	softirq_enter();
 
 	cpu = smp_processor_id();
 restart:
@@ -211,6 +247,8 @@ restart:
 		pending >>= 1;
 	} while (pending);
 
+	softirq_delay_exec();
+
 	local_irq_disable();
 
 	pending = local_softirq_pending();
@@ -220,7 +258,7 @@ restart:
 	if (pending)
 		wakeup_softirqd();
 
-	trace_softirq_exit();
+	softirq_exit();
 
 	account_system_vtime(current);
 	_local_bh_enable();
diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
index 280332c..1aa7351 100644
--- a/lib/locking-selftest.c
+++ b/lib/locking-selftest.c
@@ -157,11 +157,11 @@ static void init_shared_classes(void)
 #define SOFTIRQ_ENTER()				\
 		local_bh_disable();		\
 		local_irq_disable();		\
-		trace_softirq_enter();		\
+		softirq_enter();		\
 		WARN_ON(!in_softirq());
 
 #define SOFTIRQ_EXIT()				\
-		trace_softirq_exit();		\
+		softirq_exit();		\
 		local_irq_enable();		\
 		local_bh_enable();
 
diff --git a/net/core/sock.c b/net/core/sock.c
index 0620046..c8745d1 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -213,6 +213,8 @@ __u32 sysctl_rmem_default __read_mostly = SK_RMEM_MAX;
 /* Maximal space eaten by iovec or ancilliary data plus some space */
 int sysctl_optmem_max __read_mostly = sizeof(unsigned long)*(2*UIO_MAXIOV+512);
 
+static void sock_readable_defer(struct softirq_delay *sdel);
+
 static int sock_set_timeout(long *timeo_p, char __user *optval, int optlen)
 {
 	struct timeval tv;
@@ -1074,6 +1076,7 @@ struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
 #endif
 
 		rwlock_init(&newsk->sk_dst_lock);
+		softirq_delay_init(&newsk->sk_delay, sock_readable_defer);
 		rwlock_init(&newsk->sk_callback_lock);
 		lockdep_set_class_and_name(&newsk->sk_callback_lock,
 				af_callback_keys + newsk->sk_family,
@@ -1691,6 +1694,43 @@ static void sock_def_readable(struct sock *sk, int len)
 	read_unlock(&sk->sk_callback_lock);
 }
 
+/*
+ * helper function called by softirq_delay_exec(),
+ * if inet_def_readable() queued us.
+ */
+static void sock_readable_defer(struct softirq_delay *sdel)
+{
+	struct sock *sk = container_of(sdel, struct sock, sk_delay);
+
+	sdel->next = NULL;
+	/*
+	 * At this point, we dont own a lock on socket, only a reference.
+	 * We must commit above write, or another cpu could miss a wakeup
+	 */
+	smp_wmb();
+	sock_def_readable(sk, 0);
+	sock_put(sk);
+}
+
+/*
+ * Custom version of sock_def_readable()
+ * We want to defer scheduler processing at the end of do_softirq()
+ * Called with socket locked.
+ */
+void inet_def_readable(struct sock *sk, int len)
+{
+	if (running_from_softirq()) {
+		if (softirq_delay_queue(&sk->sk_delay))
+			/*
+			 * If we queued this socket, take a reference on it
+			 * Caller owns socket lock, so write to sk_delay.next
+			 * will be committed before unlock.
+			 */
+			sock_hold(sk);
+	} else
+		sock_def_readable(sk, len);
+}
+
 static void sock_def_write_space(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
@@ -1768,6 +1808,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 		sk->sk_sleep	=	NULL;
 
 	rwlock_init(&sk->sk_dst_lock);
+	softirq_delay_init(&sk->sk_delay, sock_readable_defer);
 	rwlock_init(&sk->sk_callback_lock);
 	lockdep_set_class_and_name(&sk->sk_callback_lock,
 			af_callback_keys + sk->sk_family,
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 05b7abb..1cc0907 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1342,6 +1342,12 @@ void udp_destroy_sock(struct sock *sk)
 	release_sock(sk);
 }
 
+static int udp_init_sock(struct sock *sk)
+{
+	sk->sk_data_ready = inet_def_readable;
+	return 0;
+}
+
 /*
  *	Socket option code for UDP
  */
@@ -1559,6 +1565,7 @@ struct proto udp_prot = {
 	.connect	   = ip4_datagram_connect,
 	.disconnect	   = udp_disconnect,
 	.ioctl		   = udp_ioctl,
+	.init		   = udp_init_sock,
 	.destroy	   = udp_destroy_sock,
 	.setsockopt	   = udp_setsockopt,
 	.getsockopt	   = udp_getsockopt,
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 84b1a29..1a9f8d4 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -960,6 +960,12 @@ void udpv6_destroy_sock(struct sock *sk)
 	inet6_destroy_sock(sk);
 }
 
+static int udpv6_init_sock(struct sock *sk)
+{
+	sk->sk_data_ready = inet_def_readable;
+	return 0;
+}
+
 /*
  *	Socket option code for UDP
  */
@@ -1084,6 +1090,7 @@ struct proto udpv6_prot = {
 	.connect	   = ip6_datagram_connect,
 	.disconnect	   = udp_disconnect,
 	.ioctl		   = udp_ioctl,
+	.init 		   = udpv6_init_sock,
 	.destroy	   = udpv6_destroy_sock,
 	.setsockopt	   = udpv6_setsockopt,
 	.getsockopt	   = udpv6_getsockopt,